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Your process looks pretty good, with the exception of MAAS cleaning for 10 HOURS? Was that a typo?
I usually do 600, 800, 1000, 1500, 2000 all on the drum sander, dry. (I'll have to see about...

Alter the direction of your grit passes just slightly. This makes it easier to see when you are ready to proceed to the next grit. So at 400 maybe go straight up and down. At 600 go at a slight angle...

I would start over on the whole blade. Spot removing here and there will leave the blade looking beat up. You want consistency across the whole blade. Any spot removal will result in waviness, and...

Personally I prefer sandpaper. There's benefits and drawbacks to either method.
600 grit should be about as low as you have to go, that is a big heavy blade, so 400 wouldn't be out of the question...

The pits along the edge don't look too bad from the pics I see. They don't look deep enough to affect the cutting edge, but only honing will tell for sure.
Are you going to hone it?
Great job so...